1. Field of the Invention
The present invention relates to methods and apparatus for eliminating static electricity in a clean room, using an ion generator. More particularly, the present invention relates to techniques of preventing the generation of fine grains which would deposit collectively on the surfaces of electrodes of an ion generator, and resplash to contaminate the clean room.
2. Related Art
Generally, factories which manufacture semiconductors are in low humidity environments with a relative humidity of about 40%. Plastic containers which are used to carry semiconductor elements and wafers enclosed therein have high electrical resistance and are likely to be electrically charged. Therefore, static electricity will be generated in the clean room. This static electricity causes dust to be deposited on the surface of wafers and hence the resulting products would be defective. Especially, the density of the semiconductor devices has recently being increased. Thus, if only a little dust deposits on the surface of a wafer, it would cause a pattern of the semiconductor device to be defective to thereby adversely affect the characteristic of the semiconductor device. Thus, semiconductor devices must be manufactured in a clean room with a degree of cleanliness close to a dust-free state.
The electromagnetic waves generated when static electricity can destroy ICs on wafers and semiconductor devices, thereby reducing product yield. Especially, as the degree of integration of semiconductor devices increases, their electrostatic resistance is decreased. Such a hindrance to manufacture due to static electricity is a big problem.
For the above reason, an ion generator is used to eliminate static electricity in the clean room. It applies high voltage across its electrodes to cause a discharge to thereby generate ions which are used to neutralize and eliminate the electric charges on the object.
One example of such ion generators is shown in FIG. 4. A clean room 1 has on its ceiling a high performance filter 5 through which clean air is fed into the room, and an ion generator 2 which eliminates static electricity. Ion generator 2 applies positive and negative high voltages to positive and negative needle electrodes 3, respectively, to generate a corona discharge. This changes the air around the needle electrodes 3 to positive and negative ions 4 which are carried by a flow of air from filter 5 to thereby neutralize opposite-polarity electric charges, respectively, on an object 6 with ions 4.
According to such ion generator 2, semiconductors which have high electrical resistance and which are difficult to leak electric charges by grounding can be neutralized electrically.
It is known that fine grains would be splashed from the electrodes 3 of the ion generator. These grains would contaminate the air in the clean room and hence be a hindrance to the manufacture of semiconductors.
The following causes of the splashing of the fine grains are known:
(a) Fine grains of the electrodes 3 produced by its wear are splashed; and
(b) The SiO.sub.2 fine grains in the air which are not eliminated by filter 5 deposit on the electrodes 3. If these grains are gathered to become particular larger ones, they would be resplashed.
Concerning the wear of the electrodes 3, electrodes the wear rate of which is decreased have been provided by improving the electrode materials.
However, no measures have been established against the resplashing of SiO.sub.2 fine grains. Even if fine grains with a size of 0.03 .mu.m or more (measurable at present) are removed by the filter 5 from the air which is fed into the clean room 1, fine grains with a size of 0.03 -0.1 .mu.m are splashed from the needle electrodes 3 in ion generator 2. It is clarified by analysis that these fine grains are of SiO.sub.2. In an uneven field such as that present around the ion generator electrodes, neutral (polarized) grains are drawn by a gradient force toward a higher field strength and deposit on the electrodes now under corona discharge. Therefore, the fine grains with a size of 0.005 .mu.m (hereinafter referred to as superfine grains) which have passed through the filter 5 are captured and collected by the needle electrodes 3 into fine grains with a size of 0.03-0.1 .mu.m, which would be resplashed in the clean room. The splashed grains would contaminate the surface of the wafers, etc. Therefore, it is very difficult to use the ion generator in a high purification degree clean room which is required to eliminate fine grains on the order of 0.1 .mu.m.
It is an object of the present invention to provide a method and apparatus for eliminating electrical charges in the clean room where no SiO.sub.2 fine grains deposit on the ion generating electrodes and hence no dust due to resplashing of such SiO.sub.2 grains is produced when clean air which contains no SiO.sub.2 fine grains is fed to the vicinity of the ion generating electrodes and static electricity in the clean room is eliminated.
It is another object of the present invention to purify the air fed to the vicinity of the ion generating electrodes by causing superfine grains which cannot be captured by the filter to deposit on the discharging electrodes so as to be larger grains which are captured with the filter, and then capturing the larger grains with the filter.
It is a further object of the present invention to purify the air fed to the vicinity of ion generating electrodes by causing superfine grains which cannot be captured with the filter to pass through pure water to thereby cause the grains to be captured with the pure water.
It is a still further object of the present invention to provide a method and an apparatus for removing electric charges in the clean room which is capable of maintaining a high degree of cleanliness, causing the ions generated by an ion generator in the clean room to neutralize the electric charges to thereby eliminate a possible obstacle to the manufacture due to static electricity and hence to ensure the manufacture of a high density semiconductor device.